Fossils and Evolution Slides

About Fossils

Fossils only occur in sedimentary rocks. Well, most
of the time. This is a mold of a tree trunk from a lava
flow in Hawaii. Now it's no particular news to know there
were palm trees in Hawaii a few centuries ago, but a
similar mold from a Silurian lava flow would be very
significant indeed.

There are a lot of things that look like fossils but
are not. The nodule on the right is a concretion,
a structure formed where the cement in the rock was a bit
different from its surroundings. Concretions can
sometimes form around fossils (you will see an example
later), but the concretion itself is not a fossil. On the
left, the fern-like patterns are dendrites, formed
by crystals of manganese oxide and other minerals, very
much the way similar patterns form in frost on a window
pane. Structures that superficially resemble fossils are
called pseudofossils.

This structure looks something like a shell, with
fine radiating lines, but it's another pseudofossil,
actually a variety of dendrite that formed on a fine
crack in the rock. In general, if something looks like it
might be a fossil, it probably isn't. It's only a
fossil if it's obvious beyond doubt.

This looks like a vaguely erotic sculpture, but
actually it's a very large and ornate concretion.

Having seen there are things that look like fossils
but are not, it's a bit easier to understand that some
people once seriously questioned whether fossils really
were the remains of living things. Some held they were
natural but inorganic, others that they were supernatural
to test our faith or tempt us. One advocate
of the inorganic school was Johannes Beringer, who
collected the specimens here, now on display in the
museum at Wurzburg, Germany.

Beringer published this elaborate book to advance his
theories. Unfortunately, even a cursory glance at
Beringer's specimens shows that they're fakes. Two colleagues, who
despised Beringer's pompous behavior, planted abundant fakes at Beringer's
real fossil locality. For the full story, see Stephen Jay
Gould's "The lying
stones of Wurzburg and Marrakech," in the April 1998
issue of Natural History.

Kingdom Protista

These single-celled organisms are free-swimming and
build calcareous shells. The disks, a centimeter in
diameter, are the great blue whales of Kingdom Protista.
The small spindle-shaped shells make up the limestone of
the Pyramids. The Greek traveler Herodotus was told that
they were petrified grains of wheat eaten by the workers
who built the Pyramids (telling a good yarn for the
tourists is an ancient craft!)

Single-celled plant-like organisms are the algae,
seen here in Lake Winnebago (yummy!) Free-swimming forms
color the water green while filamentous colonial forms
grow from the rocks.

During the Precambrian, the tidal flats of the world
were dominated by colonial algae called stromatolites
that built mound-like colonies. The appearance of bottom
grazing organisms ended the dominance of the
stromatolites, though they are found in a few places
today. This fossil is at High Cliff State Park.

Kingdom Animalia

The simplest organisms with distinctly different
types of cells for specialized functions are the sponges
and their relatives. A modern sponge is at upper right.
Thw two fossils are organisms of uncertain affinity that
are thought to be sponge-like. The banded specimen occurs
as water-worn pebbles on the Lake Michigan shore, and is
sometimes called "Lake Michigan Agate." Its
scientific name is stromatoporoid.

The coelenterates are the simplest organisms that
have three distinct cellular layers, typical of all more
complex animals. Jellyfish like these are 98% water and
are rarely fossilized for obvious reasons.

The coelenterates called corals are abundant in the
geologic record.

Brachiopods were abundant in the Paleozoic and still
exist in a few places. They superficially look like clams
but are completely different in their internal organs.
Best way to tell them apart: clams have asymmetrical
shells but the two mating shells are (usually) identical.
Brachiopods have symmetrical shells but the two mating
shells are different.

Mollusks are abundant fossils. During the Paleozoic, cephalopods,
relatives of the nautilus, squid and octopus, were
abundant. Some had coiled shells like a nautilus, others
had long straight conical shells.

The most abundant organisms in terms of number of
species are the arthropods, including insects, spiders,
scorpions, and crustaceans. Most of the fossils shown
here are trilobites, which were abundant throughout the
Paleozoic but then became extinct.

Echinoderms include starfish, sand dollars and sea
urchins. Crinoids are something like starfish on a stalk.
They were abundant in the Paleozoic and still live in
some deep ocean areas.

Chordates are organisms with a stiffening rod along
one side. They include mammals, birds, reptiles,
amphibians, several classes of fish, and several classes
of simpler organisms, including things one would scarcely
guess at first sight were chordates.

A few chordate fossils. If you find a piece of rock
with a white lump in it, you can be absolutely certain it
is not a bone Real fossil bones are dark brown
because of mineralization.

Dinosaur tracks near Denver, Colorado.

Probably the most dramatic intermediate fossil form. Archaeopteryx
("ancient winged one") has feathers but its
skeleton is purely reptilian with four fully-formed paws,
a bony vertebrate tail and jaws with teeth.

Few subjects have been as rich a source of hoaxes as
the origins of humans. The photos purportedly show
ancient giant fossil footprints but the author made the
mistake of putting a real foot in the photograph. Real
feet are narrower and the line acoss the toes slants more
sharply back than the fakes.

In the 19th century, a
celebrated fake fossil of a petrified giant human was
exhibited in Cardiff, New York and came to be called the
"Cardiff Giant." Showman P.T. Barnum tried to
buy it for his museum. When the owner refused to sell,
Barnum decided a fake is a fake is a fake, had one carved,
and put the fake fake on display anyway!

Kingdom Fungi

Fungi differ so fundamentally from other plants that
they are now considered a separate kingdom. Since they
lack hard parts they are rarely fossilized.

Kingdom Plantae

Spore-bearing plants include the mosses (shown here)
as well as several other groups of plants.

Club mosses are small spore-bearing plants today but
their Paleozoic relatives grew to tree size.

Horsetails are closely related to club mosses.

Ferns are the best-known spore-bearing plants.

During the Paleozoic, ferns grew to tree size. There
are still some tree ferns in the world. These, near
Kilauea in Hawaii, are six inches thick and 30-40 feet
high.

Fossils of ferns and their allies. The scaly specimen
is a piece of bark from a tree rather like a giant
horsetail. It dropped branches as it grew, leaving the
distinctive scar pattern on the bark

The simplest true seed plants are the conifers.

Metasequoia, shown here, was abundant
worldwide in the Mesozoic but thought to be extinct until
it was discovered growing in a remote region in China. A
redwood forest, not a jungle, was the world of the
dinosaurs.

Angiosperms, or broad-leaved plants, are the most
complex and abundant plants. This maple tree is an
example.

These fossil leaves from Illinois occur within
concretions. The organic matter of the leaf changes the
chemistry of the surrounding rock enough to cause the
precipitation of a different kind of cement. However, the
concretion itself is not a fossil.

How easy is it to make a fossil? These leaf prints on
a sidewalk show that it takes very little time to leave
an organic film behind.

References

The true story of the Beringer affair is rather different from the version
presented in many textbooks. The most easily accessible reference is:

Stephen Jay Gould, "The lying
stones of Wurzburg and Marrakech," Natural History,
April 1998. Gould bases his account of the Beringer affair on: